Rat Alpha-lactalbumin gene expression in the mammary gland is under the influence of insulin and hydrocortisone. Prolactin, however, enhances this expression and can be replaced by a factor present in bovine serum albumin. We have isolated rat Alpha-lactalbumin gene and constructed various plasmids carrying the regulatory sequences of the gene. We are presently studying the involvement of the individual hormones in the expression of these plasmids in the transfected mammalian cells.